This invention relates generally to steel tubing, and, more specifically, to a new process of making steel tubing utilizing full hard, cold rolled sheet having a chemistry compliant with industry standard S.A.E. J 403 grade 1026 welded by the electric resistance welding method.
The tubing made in accordance with the invention is intended to replace current drawn-over-mandrel (DOM) tube used in the manufacture of products requiring high torsional strength, such as propeller shafts and the like. DOM tube is made by drawing an electric resistance welded tube over a mandrel to stretch the tube and give it a specific ID. The tube typically requires thermal and/or cold drawing in order to achieve the desired strength levels. The DOM process is relatively expensive and requires considerable labor. See U.S. Pat. No. 5,411,198.
The process of the present invention comprises the steps of hot rolling steel sheet, coiling the sheet at a temperature in a range of from about 1000-1200xc2x0 F., subjecting the sheet to cold reduction in the range of about 15-30%, forming the sheet into a tube, and joining the edges of the tube by electric resistance welding. In the preferred process, the steel has a chemistry compliant with industry standard S.A.E. J 403 grade 1026 comprising 0.22-0.28 C, 0.60-0.90 Mn, 0.035 maximum S, and 0.035 maximum P.
Tubing made in accordance with the present invention can be made at a lower cost than DOM tubing. It does not require any post-welding thermal treatment processing and does not undergo any cold drawing processing. The high strength levels that are achieved are a result of the specified thermal-mechanical processing of the hot rolled sheet, i.e. coiling at the specified temperatures, and the controlled amount of cold reduction of the sheet. Tubing made in the manner described have yield strengths at or above 75,000 psi, as measured in tension in a longitudinal orientation, which is necessary to meet the torque capacity required for propeller shaft applications and the like.
An added advantage is that the new process is versatile in the tubing sizes that can be made. More specifically, tube can be produced to any one of (1) an outside diameter and wall thickness requirement, (2) an inside diameter and wall thickness requirement, or (3) an outside diameter and inside diameter requirement. Other features and advantages of the invention will be apparent from the following detailed description.